1. Field of the Invention
The present invention relates to the art of well drilling and earth boring. More particularly, the invention relates to methods and apparatus for perforating wellbore casing, casing liner and/or fracturing well production zones.
2. Description of Related Art
After the actual drilling of a borehole into the earth, the borehole shaft is often prepared for long term fluid production by a series of steps and procedures that are collectively characterized by the art as “completion.” Among these numerous procedures is the process of setting a casing, usually steel, within the borehole to line the shaft wall with a stable, permanent barrier. This casement is often secured by cement that is pumped into the annulus between the outside diameter of the casing and the inside diameter of the raw shaft wall.
While the casing stabilizes the shaft wall, it also seals the fluids within the earth strata that have been penetrated by the borehole from flowing into the borehole. The borehole inflow of some of the fluids is the desired objective of making the borehole in the first place. To selectively open the casing to such fluid flow, the casing wall is often penetrated in the region of a fluid production zone by shaped charge explosives or “bullets”. In the case of shaped charge explosives, the gaseous product of decomposing explosive material is focused linearly as a high temperature plasma to burn a perforation through the casing wall. Numerous of these charges are loaded into tubular “guns”, usually in a helical pattern along and around the gun tube axis for positioning within the wellbore at the desired location. The line of discharge from the gun is radial from the gun tube axis.
By traditional prior art procedure, the tubular gun may be releasably secured to the end of a wireline or coiled tube for running into the well. When the gun has been located at the desired depth, the gun is secured to the casing or casing liner bore wall by radially expandable slips, for example. This setting or anchoring procedure is essential to substantially center the gun within the casing bore for radially uniform penetration. In some cases, the slips are releasable from the casing to facilitate removal of the gun assembly from the casing bore in the event that need arises: either before of after firing.
Subsequent to the prior art perforation procedure, the production tubing is run into the well and set. Often, setting of the production tubing also includes a production packer around the production tubing to seal the well annulus around the tubing above the perforation zone.
The downhole environment of a deep earth boring is frequently hostile to the extreme. The borehole is usually filled with a mixture of drilling fluids, water and crude petroleum. At such depths, the bottom hole pressures may be in the order of tens of thousands of pounds per square inch and at hundreds of degrees Celsius temperature. Consequently, by the time the perforating gun arrives at the desired perforation location, the ignition system, the explosives or the propellant charges are sometimes compromised to the extent that discharge fails to occur on command. In anticipation of such contingencies, provision is often made for unrelated alternative firing systems. If all else fails, the defective gun must be withdrawn from the well and repaired or replaced and returned.
As a further consideration, many of the well completion steps require specific tools that are operatively secured within the length of a pipe or tubing work string and deposited into the wellbore from the surface. Placement of a completion tool on downhole location may require many hours of extremely expensive rig time and skilled labor. The full cycle of downhole tool placement and return is termed in the art as “a trip.”
At the present state of art, many of the necessary well completion tools are assembled collectively on a single work string and run into the wellbore together for the purpose of accomplishing as many of the several completion steps in as few “trips” as possible. There could be many advantages, therefore, for including the perforation gun at the end of a completion tube having a well production packer set above the gun prior to discharge. In a single trip, the well could be perforated, fractured, packed and produced. On the negative side, however, should the gun misfire, it would be necessary to disengage the production packer and withdraw the entire work string to repair or replace the perforation gun.
Comparatively, tools and instruments suspended from drum reeled “wirelines” are run into and out of a wellbore quickly and efficiently. There are advantages, therefore, in a well completion procedure that could position, secure, remove and/or replace a perforation gun or other such tool entirely by wireline. On the other hand, state-of-the-art wireline perforation is substantially a single purpose operation. The well is first perforated and, subsequently, the production packer is set.
Some completion assemblies connect the gun to the work string in such a manner that releases the spent gun tube to free fall further down the wellbore below the perforated production zone. In some cases, this gun release function may be desirable. In other cases, especially when additional drilling may be contemplated, the spent gun becomes downhole “junk” and must be extracted by a fishing operation.
It is, therefore, an object of the present invention to provide a means and method for securing a perforating gun to the end of a completion or production tube for alternative operational modes. In one mode, the gun may automatically disconnect from the work string when the gun is discharged and free fall from the perforation zone. In another operational mode, the gun may be tethered to a wireline and withdrawn from the well after discharge.
Another object of the invention is provision of a perforation gun assembly that may be lowered into a well along a work string tube bore at the end of a wire line, secured to the tube bore at the desired position and discharged. In the event of malfunction, the gun may, by wireline, be disconnected from the work string tube, withdrawn for repair, and returned by wireline.